Popular Science Monthly/Volume 1/September 1872/Popular Geology
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GEOLOGY is the science which explains to us the rind of the earth; of what it is made; how it has been made. It tells us nothing of the mass of the earth. That is, properly speaking, an astronomical question. If I may be allowed to liken this earth to a fruit, then astronomy will tell us—when it knows—how the fruit grew, and what is inside the fruit. Geology can only tell us at most how its rind, its outer covering, grew, and of what it is composed; a very small part, doubtless, of all that is to be known about this planet.
But, as it happens, the mere rind of this earth-fruit, which has, countless ages since, dropped, as it were, from the Bosom of God, the Eternal Fount of Life—the mere rind of this earth-fruit, I say, is so beautiful and so complex, that it is well worth our awful and reverent study. It has been well said, indeed, that the history of it, which we call geology, would be a magnificent epic poem, were there only any human interest in it; did it deal with creatures more like ourselves than stones, and bones, and the dead relics of plants and beasts. Whether there be no human interest in geology; whether man did not exist on the earth during ages which have seen enormous geological changes, is becoming more and more an open question.
But meanwhile all must agree that there is matter enough for interest—nay, room enough for the free use of the imagination, in a science which tells of the growth and decay of whole mountain-ranges, continents, oceans, whole tribes and worlds of plants and animals.
And yet it is not so much for the vastness and grandeur of those scenes of the distant past, to which the science of geology introduces us, that I value it as a study, and wish earnestly to awaken you to its beauty and importance. It is because it is the science from which you will learn most easily a sound scientific habit of thought. I say most easily; and for these reasons. The most important facts of geology do not require, to discover them, any knowledge of mathematics or of chemical analysis; they may be studied in every bank, every grot, every quarry, every railway-cutting, by any one who has eyes and common-sense, and who chooses to copy the late illustrious Hugh Miller, who made himself a great geologist out of a poor stone-mason. Next, its most important theories are not, or need not be, wrapped up in obscure Latin and Greek terms. They may be expressed in the simplest English, because they are discovered by simple common-sense. And thus geology is (or ought to be), in popular parlance, the people's science—the science by studying which, the man ignorant of Latin, Greek, mathematics, scientific chemistry, can yet become—as far as his brain enables him—a truly scientific man.
But how shall we learn science by mere common-sense?
First, always try to explain the unknown by the known. If you meet something which you have not seen before, then think of the thing most like it which you have seen before; and try if that which you know explains the one will not explain the other also. Sometimes it will; sometimes it will not. But, if it will, no one has a right to ask you to try any other explanation.
Suppose, for instance, that you found a dead bird on the top of a cathedral-tower, and were asked how you thought it had got there. You would say, "Of course, it died up here." But if a friend said: "Not so; it dropped from a balloon, or from the clouds;" and told you the prettiest tale of how the bird came to so strange an end, you would answer: "No, no; I must reason from what I know. I know that birds haunt the cathedral-tower; I know that birds die; and therefore, let your story be as pretty as it may, my common-sense bids me take the simplest explanation, and say—it died here." In saying that, you would be talking scientifically. You would have made a fair and sufficient induction (as it is called) from the facts about birds' habits and birds' deaths which you knew.
But suppose that when you took the bird up you found that it was neither a jackdaw, nor a sparrow, nor a swallow, as you expected, but a humming-bird. Then you would be adrift again. The fact of it being a humming bird would be a new fact which you had not taken into account, and for which your old explanation was not sufficient: and you would have to try a new induction—to use your common-sense afresh—saying, "I have not to explain merely how a dead bird got here, but how a dead humming-bird."
And now, if your imaginative friend chimed in triumphantly with, "Do you not see that I was right after all? Do you not see that it fell from the clouds? That it was swept away hither, all the way from South America, by some southwesterly storm, and, wearied out at last, dropped here to find rest, as in a sacred place?" what would you answer? "My friend, that is a beautiful imagination: but I must treat it only as such, as long as I can explain the mystery more simply by facts which I do know. I do not know that humming-birds can be blown across the Atlantic alive. I do know that they are actually brought across the Atlantic dead; are stuck in ladies' hats. I know that ladies visit the cathedral: and, odd as the accident is, I prefer to believe, till I get a better explanation, that the humming-bird has simply dropped out of a lady's hat." There, again, you would be speaking common-sense; and using, too, sound inductive method; trying to explain what you do not know from what you do know already.
Now, I ask of you to employ the same common-sense when you read and think of geology.
It is very necessary to do so. For in past times men have tried to explain the making of the world around them, its oceans, rivers, mountains, and continents, by I know not what of fancied cataclysms and convulsions of Nature; explaining the unknown by the still more unknown, till some of their geological theories were no more rational, because no more founded on known facts, than that of the New Zealand Maories, who hold that some god, when fishing, fished up their islands out of the bottom of the ocean. But a sounder and wiser school of geologists now reigns; the father of whom, in England at least, is the venerable Sir Charles Lyell. He was almost the first of Englishmen who taught lis to see—what common-sense tells us—that the laws which we see at work around us now have been most probably at work since the creation of the world; and that whatever changes may seem to have taken place in past ages, and in ancient rocks, should be explained, if possible, by the changes which are taking place now in the most recent deposits—in the soil of the field.
And in the last 40 years—since that great and sound idea has become rooted in the minds of students, and specially of English students—geology has thriven and developed, perhaps more than any other science; and has led men on to discoveries far more really astonishing and awful than all fancied convulsions and cataclysms.
I have planned this series of papers, therefore, on Sir Charles Lyell's method. I have begun by trying to teach a little about the part of the earth's crust which lies nearest us, which we see most often—namely, the soil; intending, if my readers do me the honor to read the papers which follow, to lead them downward, as it were, into the earth; deeper and deeper in each paper, to rocks and minerals which are probably less known to them than the soil in the fields. Thus you will find I shall lead you, or try to lead you on, throughout the series, from the known to the unknown, and show you how to explain the latter by the former. Sir Charles Lyell has, I see, in the new edition of his "Student's Elements of Geology," begun his book with the uppermost, that is, newest strata, or layers; and has gone regularly downward in the course of the book to the lowest or earliest strata; and I shall follow his plan.
I must ask you meanwhile to remember one law or rule, which seems to me founded on common-sense, namely, that the uppermost strata are really almost always the newest; that when two or more layers, whether of rock or earth—or indeed two stones in the street, or two sheets on a bed, or two books on a table—any two or more lifeless things, in fact, lie one on the other, then the lower one was most probably put there first, and the upper one laid down on the lower. Does that seem to you a truism? Do I seem almost impertinent in asking you to remember it? So much the better. I shall be saved unnecessary trouble hereafter.
But some one may say, and will have a right to say, "Stop—the lower thing may have been thrust under the upper one." Quite true: and therefore I said only that the lower one was most probably put there first. And I said "most probably," because it is most probable that in Nature we should find things done by the method which costs least force, just as you do them. I will warrant that, when you want to hide a thing, you lay something down on it ten times for once that you thrust it under something else. You may say: "What? When I want to hide a paper, say, under the sofa-cover, do I not thrust it under?" No, you lift up the cover, and slip the paper in, and let the cover fall on it again. And so, even in that case, the paper has got into its first place.
Now, why is this? Simply because in laying one thing on another you only move weight. In thrusting one thing under another, you have not only to move weight, but to overcome friction. That is why you do it, though you are hardly aware of it: simply because so you employ less force, and take less trouble.
And so do clays and sands and stones. They are laid down on each other, and not thrust under each other, because thus less force is expended in getting them into place.
There are exceptions. There are cases in which Nature does try to thrust one rock under another. But to do that she requires a force so enormous, compared with what is employed in laying one rock on another, that (so to speak) she continually fails; and, instead of producing a volcanic eruption, produces only an earthquake. Of that I may speak hereafter, and may tell you, in good time, how to distinguish rocks which have been thrust in from beneath, from rocks which have been laid down from above, as every rock between London and Birmingham or Exeter has been laid down. That I only assert now. But I do not wish you to take it on trust from me. I wish to prove it to you as I go on, or, to do what is far better for you, to put you in the way of proving it for yourselves, by using your common-sense.
At the risk of seeming prolix, I must say a few more words on this matter. I have special reasons for it. Until lean get you to "let your thoughts play freely: round this question of the superposition of soils and rocks, there will be no use in my going on with these papers.
Suppose, then (to argue from the known to the unknown), that you were watching men cleaning out a pond. Atop, perhaps, they would come to a layer of soft mud, and under that to a layer of sand. Would not common-sense tell you that the sand was there first, and that the water had laid down the mud on the top of it? Then, perhaps, they might come to a layer of dead leaves. Would not common-sense tell you that the leaves were there before the sand above them? Then, perhaps, to a layer of mud again. Would not common-sense tell you that the mud was there before the leaves? And so on down to the bottom of the pond, where, lastly, I think common-sense would tell you that the bottom of the pond was there already, before all the layers which were laid down on it. Is not that simple common-sense?
Then apply that reasoning to the soils and rocks in any spot on earth. If you made a deep boring, and found, as you would in many parts of this kingdom, that the boring, after passing through the soil of the field, entered clays or loose sands, you would say the clays were there before the soil. If it then went down into sandstone, you would say—would you not?—that sandstone must have been here before the clay; and however thick even thousands of feet—it might be, that would make no difference to your judgment. If next the boring came into quite different rocks, into a different sort of sandstone and shales, and among them beds of coal, would you not say, "These coal-beds must have been here before the sandstones?" And, if you found in those coal-beds dead leaves and stems of plants, would you not say: "Those plants must have been laid down here before the layers above them, just as the dead leaves in the pond were?"
If you then came to a layer of limestone, would you not say the same? And if you found that limestone full of shells and corals, dead, but many of them quite perfect, some of the corals plainly in the very place in which they grew, would you not say, "These creatures must have lived down here before the coal was laid on top of them?" And if, lastly, below the limestone, you came to a bottom-rock quite different again, would you not say, "The bottom-rock must have been here before the rocks on the top of it?"
And if that bottom-rock rose up a few miles off, 2,000 feet, or any other height, into hills, what would you say then? Would you say: "Oh, but the rock is not bottom-rock; is not under the limestone here, but higher than it. So perhaps in this part it has made a shift, and the highlands are younger than the lowlands; for see, they rise so much higher?" Would not that be about as wise as to say that the bottom of the pond was not there before the pond-mud, because the banks round the pond rose higher than the mud?
- From advance sheets of Prof. Kingsley's excellent little book entitled "Town Geology."